1. Field of the Invention
The invention relates to a DC-DC power converter. More specifically, the invention relates to a converter in which the voltage across the main switch due to the leakage inductance of the transformer is clamped and the leakage energy of the transformer is recycled instead of being is dissipated by the circuit so as to improve the circuit efficiency.
2. Description of Related Art
A well-known conventional DC/DC flyback converter is shown in FIG. 1, where Lk 10 is the leakage inductance of the transformer T 12. The typical switching waveforms of FIG. 1 are shown in FIG. 2. When switch S 14 is turned off at t2, the leakage current charges the parasitic output capacitance of switch S 14 (output capacitance of S is not shown in FIG. 1), which causes a high voltage spike across switch S 14. After the leakage energy is completely released, the voltage across switch S 14 reaches its steady-state value. As a result, a high voltage rating for switch S 14 is required.
To eliminate this voltage spike, a number of circuit topologies have been reported in the literature. Among them, the R-C-D snubber, shown in FIG. 3 is one of the most popular ways to minimize the voltage spike as shown in FIG. 2. The snubber circuit consists of diode D120, capacitor Cs 22 and resistor Rs 24. When switch S 14 is turned off, the leakage current flows through diode D120 and charges capacitance Cs 22. If capacitance Cs 22 is relatively large, the voltage across Cs 22 does not change so as to clamp the voltage. In this case, the leakage energy of the transformer is first charged to Cs 22 and then is dissipated by the resistor Rs 24. As a result, the voltage clamp is achieved at the expense of low conversion efficiency.
The invention is a DC-DC converter in which the voltage across the main switch due to the leakage inductance of the transformer is clamped and the leakage energy of the transformer is recycled instead of being dissipated by the circuit so as to improve the circuit efficiency. The DC-DC converter has a voltage source which is connected to a diode. A first transformer primary winding is in series with a first capacitor. This winding and capacitor are connected across the voltage source and diode. A second transformer primary winding is in series with a second capacitor. They are also connected across the voltage source and diode. The first and second transformer primary windings have first and second leakage inductances respectively.
A switch has one terminal connected to terminals of the first transformer primary winding and the first capacitor. The switch also has a second terminal connected to the terminals of the second transformer primary winding and the second capacitor. The transformer first and second primary windings and the transformer secondary winding are included in the transformer. The transformer has a magnetizing inductance providing a delivered output to the transformer secondary winding. A parallel load capacitor and load resistor are connected across the transformer secondary winding and diode.
The advantage of the inventive DC-DC converter is that the voltage across the main switch due to the leakage inductance of the transformer is clamped. In addition, the leakage energy of the transformer is recovered by charging the first and second capacitors and the delivered output by the magnetizing inductance instead of being dissipated by the circuit so as to improve the circuit efficiency. Another objective of the invention is to use as few components as possible and use only one active switch to reduce the cost.